1. Field
This invention relates to electromagnetic therapy and more particularly to applicators for applying electromagnetic energy to a treatment site.
2. State of the Art
The use of electromagnetic (EM) energy in the heating arts has been utilized for many years. In recent years EM energy has been utilized in diathermy and hyperthermia to provide therapeutic heating to diseased tissue. The application of the frequencies used have typically ranged from 100 kHz to 2450 MHz.
The use of heating for cancer therapy is commonly called hyperthermia, which is one of the intended uses of embodiments of this invention. This may be in combination with other treatments such as surgery, ionizing radiation, and chemotherapy. In hyperthermia treatment it is common to attempt heating the diseased tissue to above about 40 degrees C., but undesirable complications can occur when the maximum tissue temperature exceeds about 45 to 46 degrees C. Some of these complications include damage to healthy normal tissue, ulceration, surface blisters, and burns.
Providing hyperthermia for superficial treatment (e.g., <3 cm) of surface tissues presents a number of unique challenges. Although various techniques for deep tissue heating have been developed, many of these approaches are unsuitable for use in surface hyperthermia. For example, one approach to deep tissue heating uses phased arrays of linearly polarized antennas. Linearly polarized antennas are used in such applications because of the predictability with which the fields superimpose. Variations in the dielectric constant of the tissue (for example, between bone, muscle, fat, and tumor) can, however, cause significant variations when the electric field is parallel to an interface between tissues, making it difficult to achieve even absorption of the EM energy into the target tissue area.
Another challenge for superficial treatment is that the treatment areas can be quite large. Conforming an applicator to the complex contours of the human body can be difficult. Uneven surfaces caused by scarring or diseased tissue further complicate this problem. Prior attempts to provide large area treatment coverage have included complex mechanical arrangements to allow mechanical scanning of antennas.
Accordingly, it has proven difficult to maintain even heating distributions using prior techniques. High heat regions can result in surface burning, while low heat regions can fail to provide a therapeutically adequate level of heat. Approximately 8 to 10 percent of patients receiving hyperthermia have received burns during treatment.
An additional difficulty with electromagnetic treatment is the desire to efficiently couple the energy into the tissue being treated while minimizing radiation of energy in other directions. For example, to prevent radiation of energy out the back side of treatment applicators, a ground plane is usually placed behind the antennas. While the ground plane is effective at helping to force radiation into the tissue, it is visually opaque. Accordingly, visual monitoring of the treatment site requires removal of the treatment applicator.